The correct answer is:

The patient’s ability to supinate his left wrist should be checked.

Educational Objective:

Recognize radiographic findings indicative of posterior shoulder dislocation.

Key Point:

Posterior shoulder dislocations are the most frequently missed dislocation in the body (~ 50% missed on first presentation). If an abnormality is present on the single AP view of the shoulder, an emergency physician stands on weak grounds when missing it (especially if this leads to not ordering a complete shoulder series).

Explanation:

This patient has a posterior shoulder dislocation. Both the history of electric shock and the arm held tightly across the body are classic presentations (a postseizure patient or a patient who braced him/herself during an automobile collision are also at high risk for having a posterior shoulder dislocation). 

Posterior shoulder dislocations are the most frequently missed dislocation in the body (~ 50% missed on first presentation). Even experienced emergency clinicians can miss these, especially if the dislocation is accompanied by a shoulder fracture that seems to explain the shoulder pain and decreased range of motion. Approximately 50% of posterior shoulder dislocations are accompanied by fracture.

As a rule, a posterior shoulder dislocation is more painful than anterior dislocation, but both neurologic or vascular injuries are less common than in anterior dislocations. On examination, external rotation will be completely blocked and abduction will be severely limited. A subtle but extremely reliable sign is the fact that the patient with a posterior shoulder dislocation will never be able to supinate his or her forearm (consider documenting the presence or absence of forearm supination in the chart of every patient presenting with a shoulder injury).

The anteroposterior x-ray of a posteriorly dislocated shoulder can look completely normal and can be nondiagnostic. A lateral x-ray is required to confirm the diagnosis. However, when an abnormality is present on the AP view (as in the above patient), missing the diagnosis is more difficult to defend.

In this patient, the x-ray is indeed abnormal, but only subtly. In the anteroposterior x-ray of the normal shoulder (figure 3 below), the humeral head will produce an elliptically shaped overlap shadow with the glenoid.

By contrast, the posteriorly dislocated shoulder will either produce a shadow that looks distorted and irregular (true of this patient’s x-ray, figure 1) or will produce a "dead-normal" looking x-ray, where there is no humeral-glenoid overlap shadow. In these cases, the humeral head seems to be perfectly positioned opposite to the glenoid fossa without touching the fossa (without any elliptical shadow overlap). Ironically, these x-rays look "more" normal than the x-ray of a normal shoulder, where some degree of humeral-glenoid overlap will be present. This “dead-normal look” without any overlap between the glenoid fossa and the humeral head is a red flag that is indicative of a posterior shoulder dislocation, especially in the context of clinical signs. Figure 2 shows the widened gap, which looks deceptively normal, of a posterior shoulder dislocation.

Posterior shoulder dislocations are more difficult to reduce than anterior ones. Attempts can be made to reduce posterior dislocations under sedation in the emergency department, but often general anesthesia will be required.

Figure 1.

Posteriorly dislocated shoulder with glenoid fossa/humeral head overlap that looks distorted and irregular (compare to figure 3)

Figure 2.

Widened glenoid fossa/humeral head gap, of a posterior shoulder dislocation, which looks deceptively normal on the AP view.

Figure 3.

Normal shoulder with normal elliptical overlap of glenoid fossa and humeral head

References:

Bengtzen R, Daya M. Shoulder (Chapter 46). In: Wall R, et al, eds. Rosen’s Emergency Medicine: Concepts and Clinical Practice. 19th ed., 2018:549-568.e2.

Brady WJ. Challenging and elusive orthopedic injuries: diagnostic and treatment strategies - part I: upper extremity fractures and dislocations. Published April 26, 1999. Accessed November 1, 2022.